Kitagishi K, Ikeda-Saito M, Yonetani T
Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia 19104-6089.
J Mol Biol. 1988 Oct 20;203(4):1119-26. doi: 10.1016/0022-2836(88)90131-3.
The oxygen dissociation constants from Fe subunits in the half-ligated intermediate states of Fe-Co hybrid hemoglobins, alpha(Fe-O2)2 beta(Co)2 and alpha(Co)2 beta(Fe-O2)2, have been determined as functions of pH, temperature and inositol hexaphosphate. The oxygen dissociation rates from alpha(Fe-O2)2 beta(Co)2 are estimated to be more than 1300 s-1 for the deoxy quaternary state (T-state) and less than 3 s-1 for the oxy quaternary state (R-state) at 15 degrees C in 50 mM-Tris or Bis-Tris buffer containing 0.1 M-Cl-, while those of alpha(Co)2 beta(Fe-O2)2 are more than 180 s-1 and less than 5 s-1 for the T and R-states, respectively. The pH dependence of the oxygen dissociation rate from Fe subunits is large enough to be accounted for by the R-T transition, and implies that those half-ligated intermediate hybrids mainly exist in the R-state at pH 8.8, and in the T-state at pH 6.6, while other studies indicated that the half-ligated hybrids are essentially in the R-state at pH 7. Large activation energies of the oxygen dissociation process of 19 to 31 kcal/mol determined from the temperature dependence suggest that the process is entropy-driven.
已测定了铁 - 钴杂合血红蛋白α(Fe - O₂)₂β(Co)₂和α(Co)₂β(Fe - O₂)₂半连接中间态中铁亚基的氧解离常数，该常数是pH、温度和肌醇六磷酸的函数。在含有0.1 M - Cl⁻的50 mM - Tris或双三羟甲基氨基甲烷缓冲液中，15℃时，α(Fe - O₂)₂β(Co)₂在脱氧四级状态（T态）下的氧解离速率估计超过1300 s⁻¹，在氧合四级状态（R态）下小于3 s⁻¹，而α(Co)₂β(Fe - O₂)₂在T态和R态下的氧解离速率分别超过180 s⁻¹和小于5 s⁻¹。铁亚基的氧解离速率对pH的依赖性大到足以由R - T转变来解释，这意味着这些半连接中间杂合体在pH 8.8时主要以R态存在，在pH 6.6时以T态存在，而其他研究表明半连接杂合体在pH 7时基本上处于R态。由温度依赖性确定的19至31 kcal/mol的氧解离过程的大活化能表明该过程是由熵驱动的。
